Valve actuators may be used to control valve operations, e.g. open or closing a valve. In most cases, e.g. during normal operation of the system comprising the valve, it is wanted to perform these operations in a controlled manner. However, in case of an emergency it may be needed to automatically open or close the valve, even during loss of power. This is especially important when used as a part of an underwater valve system, where leakage of fluids may be especially harmful to the environment.
EP1210538 discloses an underwater valve actuator comprising an actuator stem that extends from a housing, which contains a sleeve. The actuator stem is driven to cause the sleeve to compress a stack of springs within the housing until bolts engage the sleeve. The springs are then locked in a compressed state. At the same time an armature on the actuator stem is attracted to an electromagnet within the housing. The actuator stem can then be driven to open or close a valve in a controlled manner without first compressing or releasing the springs. In an emergency, the electromagnet is switched off, releasing the armature thus unlocking the stack of springs. The released springs push the piston which partially retracts the actuator stem into the housing causing the valve to close.
However, this solution has several disadvantages. Due to that the motor, the bearing arrangement and the sleeve perform a translation movement during energizing and de-energizing of the stack of springs, high precision tolerances are required. Furthermore, since the motor must be moved back and forth, electrical connections to the motor need to allow movement too, which increases the risk of cable break. The stack of springs is acting directly upon the motor and the bearing arrangement, thus causing them to receive large loads. Another disadvantage is the locking means, which is supposed to directly bear the full load of the arrangement comprising the spring, the sleeve, the motor, the bearing etc. by blocking the arrangements linear movement. Consequently, it has to be dimensioned to withstand large loads, thus requiring a more expensive magnet, and perhaps also more space.
WO2004/065832 discloses a motor-controlled actuator for regulating a valve. An actuator element is moved in a rectilinear movement within two extreme positions, controlled by an electric motor, whose rotating movement is converted to the rectilinear movement by means of a threaded spindle and a nut. A spring is acting in the movement direction of the actuator element, causing the electric motor and the actuator element to move to one of the two extreme positions in case of loss of power.
A disadvantage with this design is the movable motor that causes similar problems as to those in EP1210538. Another disadvantage is the holding device that acts in the opposite direction of the spring force. In WO2004/065832 an electromagnet is described as the holding device, but the use of a latch is also suggested. However, if the valve actuator is of large dimensions, which for instance is required when used deep under water, a high force and/or high energy consumption is needed to hold the electric motor with such a design.